The use of acoustic (e.g., audible and/or ultrasonic) measurement systems in prior art downhole applications, such as logging while drilling (LWD) and wireline logging applications is well known. In one common logging operation, acoustic waveforms may be generated at one or more transmitters deployed in the borehole. The acoustic responses may then be received at an array of longitudinally spaced receivers deployed in the borehole. Acoustic logging in this manner provides an important set of borehole data and is commonly used in both LWD and wireline applications to determine compressional and shear wave velocities (also referred to in the art as slowness) of a formation. Those of ordinary skill in the art will appreciate that the terms slowness and velocity are often used interchangeably with the understanding that they are inversely related to one another and that the measurement of either may be converted to the other by simple mathematical calculations. The term velocity is used predominately herein.
As is known to those of ordinary skill in the art, acoustic LWD introduces significant difficulties as compared to acoustic wireline logging. In particular, both tool mode arrivals (signals) and drilling noise are known to affect both detection capability and efficiency and the data quality of the received waveforms. Tool mode arrivals include sonic energy that travels along the metallic tool body from the transmitter(s) to the receivers. These arrivals generally include both a fast travelling extensional wave and a slower (lower velocity) flexural wave, both of which travel along the tool body and therefore bypass the formation. Tool mode noise can therefore interfere with the determination of both compressional and shear wave velocities. Drilling noise includes vibrational energy caused by the drilling process (e.g., including the grinding action of the bit on the formation, impacts between the drill string and the borehole wall, and the circulation of drilling fluid in the tool and annulus). Drilling noise commonly has a frequency of less than or equal to about 5 kHz and therefore predominately (although not exclusively) interferes with the determination of a formation shear wave velocity.
Tool mode arrivals are readily accommodated in wireline logging operations. For example, a slotted steel housing and flexible rubber/steel segmented isolation assemblies are commonly employed to attenuate the tool mode signals. While these assemblies tend to work well in wireline operations, they are generally impractical in LWD operations since they can significantly reduce the mechanical strength of the tool body. Drilling noise is, of course, not present in wireline operations.
Numerous techniques have been employed to accommodate tool mode arrivals and drilling noise in LWD operations. For example, various mechanical isolation mechanisms are known in the art. While these mechanisms can provide some attenuation of the tool mode signals, they are limited due to mechanical strength constraints and therefore do not fully attenuate the tool modes. Signal processing algorithms have also been employed to remove the tool modes. However, in many operations significant tool mode noise remains after processing, due in part to the complicated and unpredictable downhole conditions including temperature, pressure, and drilling fluid affects as well as drilling factors including shock, vibration, and tool eccentricity in the borehole. Moreover, these mechanical and signal processing mechanisms do not tend to fully address the above described low frequency drilling noise.
One other approach has been to deploy reference receivers of various kinds internally in the logging while drilling tool body. These reference receivers are intended to sense tool mode signals and drilling noise so that they can be cancelled or otherwise accommodated. For example, U.S. Pat. No. 5,780,784 to Robbins teaches a reference receiver and adaptive filter for receiving the tool mode signal and generating a tool mode corrected signal. The reference receiver is disclosed as being deployed internally in the tool. U.S. Pat. No. 5,886,303 to Rodney discloses a configuration in which multiple reference receivers (and/or motion sensors) are deployed internally in the tool so as to cancel out unwanted noise signals. U.S. Pat. No. 6,470,275 to Dubinsky discloses the use of internally deployed reference accelerometers (rather than acoustic receivers) to cancel out tool mode signals.
While these approaches may enable the removal (or reduction) of some tool mode arrivals and/or drilling noise, there is yet room for further improvement. In particular, there is a need for an apparatus and method for substantially eliminating tool mode and drilling noise signals.